Invasive plants in a warming world
They’re out there. Little by little, a silent invasion is sweeping across the Northeast landscape, and the rest of the world for that matter. An oft overlooked but devastating ecological crisis: invasive exotic plants are exacting a toll on New England’s array of forests, fields and wetlands.
The invaders – released outside their native lands, notably Europe and Asia – got their foothold here in the early 19th century. At that time, Americans were embracing plants from across the globe for both ornamental and agricultural purposes. Now infamous for escaping their garden boundaries, the ecological havoc wrought by these plants on our natural systems is well documented.
Free from the insect and disease predators of their native countries, invasive plants can easily out-compete and displace entire native plant communities.
Let’s keep in mind why plants are crucial to life on earth. They are the only organisms capable of capturing the sun’s energy and, through photosynthesis, converting that energy into food for animals, including us. Insects play a critical role in food webs by consuming native plants, and in doing so, transferring the energy otherwise locked in plants to other animals (for example, birds and frogs) that prey upon them.
Most native insects cannot or will not eat invasive plants. When native plants are crowded out by invasives, insects — including many that are beneficial to people — are deprived of essential food sources, ultimately leading to a weakened food chain.
As native vegetation and native wildlife are inextricably linked, what was once a healthy, tightly integrated assemblage of native plants and wildlife begins to unravel. What, then, does the specter of plant invasions have to do with wetlands and global warming? Plenty. While much attention has been brought to anticipated sea level rise and increased frequency and intensity of storms due to climate change – all of which adversely impact coastal wetlands – much uncertainty remains about the fate of our inland marshes and swamps in the face of rising temperatures.
At least one variable seems a little more predictable, however: USDA plant scientists are finding that increased levels of atmospheric carbon dioxide (a primary driver of climate change) appear to favor the growth of some invasive plants over their native counterparts. If such observations hold true, we could witness an even more dramatic expansion of non-native plants blanketing our most coveted open spaces, especially wetlands.
Wetlands are particularly vulnerable to non-native plant invasions. Referred to as landscape “sinks,” most wetlands are low-lying areas where debris, sediments and water accumulate from up-gradient locations. As water flows into a wetland, it can carry invasive seeds and fragments of invasive plants from more distant locations. Woody debris and eroded sediments are also flushed into wetlands during storms, and in the process smother native vegetation. An area of smothered vegetation can create an opening in the wetland canopy, which in turn provides direct sunlight to disturbed, barren soils — all ideal conditions under which invasive plants tend to take hold.
In the face of such adversity, what’s a beleaguered wetland to do? Fortunately, federal and state agencies, nonprofit organizations and volunteer groups are stepping up to the challenge of controlling exotic invasions.
EPA has awarded grants to local groups to fight the battle. Most recently, an EPA 5-star wetland restoration grant was awarded to the Massachusetts Audubon Society for using “biocontrol agents” in the form of the Galerucella beetle to control the spread of the exotic purple loosestrife in the Great Marsh of Essex County.
This is just a start. We will need additional innovative and expanded efforts to combat invasive plants. It’s not too late to save the precious matrix of native plants, insects and animals that comprise our natural New England.
Robert Varney is administrator of the U.S. Environmental Protection Agency’s New England regional office.